1. Field of the Invention
The present invention relates in general to on-screen display circuits, and more particularly to an on-screen display circuit of the interlaced scanning type for displaying a character or picture on a screen in an interlaced scanning manner to double a definition degree.
2. Description of the Prior Art
Generally, an on-screen display (referred to hereinafter as OSD) function is to display a desired character or picture on a desired position of a screen. Referring to FIG. 1, there is shown a block diagram of a conventional OSD circuit performing the OSD function. As shown in this drawing, the conventional OSD circuit comprises a vertical counter 1 for counting a horizontal synchronous signal H-SYNC inputted thereto, a vertical decoder 2 for decoding an output signal from the vertical counter 1 and outputting the decoded signal to designate a decoded vertical address, a character decoder 3 for decoding a character address inputted thereto and outputting the decoded signal to designate a decoded horizontal address, and a font read only memory (referred to hereinafter as ROM) 4 for storing information regarding characters or pictures to be displayed on the screen and outputting font data in its location corresponding to the decoded vertical address from the vertical decoder 2 and the decoded horizontal address from the character decoder 3.
The operation of the conventional OSD circuit with the above-mentioned construction will hereinafter be described.
First, the horizontal synchronous signal H-SYNC is separated from a broadcasting signal and then applied to the vertical counter 1. Upon inputting the horizontal synchronous signal H-SYNC, the vertical counter 1 counts the inputted horizontal synchronous signal H-SYNC and, as a result of the counting operation, outputs the vertical address to the vertical decoder 2, the vertical address designating a vertical dot of a font to be displayed on the screen. The vertical decoder 2 decodes the vertical address from the vertical counter 1 and outputs the decoded signal to the font ROM 4 to designate the decoded vertical address thereof. On the other hand, upon inputting the character address, the character decoder 3 decodes the inputted character address and outputs the decoded signal to the font ROM 4 to designate the decoded horizontal address thereof. As a result, the font ROM 4 outputs the font data in its location corresponding to the decoded vertical address from the vertical decoder 2 and the decoded horizontal address from the character decoder 3.
In the above-mentioned conventional OSD circuit, the decoded vertical decoder 2 is adapted to designate the vertical address of the font to be displayed on the screen using merely the counted value of the horizontal synchronous signal H-SYNC with no consideration for an interlaced scanning manner. For this reason, in the case where the addresses of the font are designated as shown in FIG. 2A, the same character or picture is displayed in both odd and even fields of a video signal as shown in FIG. 2B. In this connection, the above-mentioned conventional OSD circuit has a disadvantage in that the same dot data is displayed in both the odd and even fields of the video signal, resulting in a reduction in a resolution by half.